1. Field of the Invention
This invention relates to apparatus for hydraulic control of a subsea device, and more particularly to hydraulic apparatus for the control of a relatively large number of subsea devices using only a pair of hydraulic pressure source lines from a surface vessel to the sea floor.
2. Description of the Prior Art
The production of oil and gas from offshore wells has developed into a major endeavor of the petroleum industry. Wells are commonly drilled several hundred or even several thousand feet below the surface of the ocean, substantially below the depth at which divers can work efficiently. As a result, the drilling and operating of a subsea well must be controlled from a surface vessel or from an offshore platform. The testing, production and shutting down of the subsea well is regulated by a subsea christmas tree which is positioned on top of the subsea wellhead. The christmas tree includes a plurality of valves having operators which are biased to a non-active position by spring returns, and it has been found convenient to actuate these operators by hydraulic fluid which is directly controlled from a surface vessel. For this purpose, a plurality of hydraulic lines are commonly run from the surface vessel to the wellhead to open and close these valves, and to actuate other devices in the well and the wellhead during installation, testing and fluid production through the subsea well equipment, and also during workover procedures being performed on the well.
In some of the prior art systems, a separate hydraulic line is run from the surface vessel to each of the hydraulically powered devices at the sea floor. Other prior art equipment uses a single hydraulic control line to control a plurality of remotely positioned hydraulically actuated devices by employing a plurality of pilot valves each arranged for actuation by a different pressure level in the hydraulic line. A pressure indicator and a pressure regulator at the surface are used to monitor the subsea pressure level to provide actuation of the desired pilot valve. Pressure difference between the subsea floor and the surface, and variation in operating characteristics of the pilot valves because of the hostile subsea environment can cause improper operation of the remotely operated pressure sensitive devices.
Still other systems use a rotary device which sequentially moves a plurality of valves through a sequence of operating steps, from the beginning of the sequence to the end and is then set to repeat this sequence. Devices which are used for rotating a plurality of positioning apparatus through a plurality of operating positions are shown in the U.S. Pat. Nos. 3,703,104, issued to Tamplen and 3,965,775, issued to Pinkesfeld.